chore: adopt shared engine maintenance improvements

[DevMando]

Update Desktop to use the shared CLI engine maintenance work in MandoCode #133. This brings the engine's clearer code organization, agent shutdown/callback fixes, and stronger regression coverage into the Desktop build.

Scope

• Update the engine submodule from the older migration revision to commit 24db4d7.
• Explicitly select Desktop's snapshot helper types where the updated engine exposes similarly named types, keeping the app build unambiguous.
• Record the engine update and compatibility fix in the unreleased changelog.

Desktop retains its existing conversation renderer and interface. This PR targets razorconsole-v0.6.0-migration so it contains only the engine update and Desktop compatibility changes.

Validation

• Desktop build succeeded with zero warnings/errors.
• All 571 Desktop tests passed.
• The pinned engine passed all 1,243 tests on both .NET 10 and .NET 8.

Merge the engine PR first. If it is squash-merged or rebased, update this submodule pin to the resulting engine commit before merging this PR.